In catalytic processes for polymerizing olefinic monomers, it is well known that the properties of the polymer powder which exits the reactor are very important. Polymer powders having a high fines content can cause problems such as accumulation of fines in the recycling units, and during the transfer and packing of the powder and sampling; also risk of accumulation of static electricity in the conveyors and the storage tanks.
It is well known to overcome some of these problems by using a catalyst with an appropriate granulometry. For example, WO 05/082962 discloses a process for polymerising olefin in the presence of a Ziegler-Natta catalyst system which is characterised in that the Ziegler-Natta catalyst has a particular particle size distribution. However, the polyethylene powders produced according to this invention still have relatively large particle sizes: 50% have a diameter (D50) of at least 400 μm.
WO 05/032714 is concerned with new catalyst systems, which are said to give a low level of fines in the resultant polymer. However the number of large particles in the polymers produced with these catalysts is relatively large, with D50 values in the Examples all greater than 230 μm, and most substantially greater.
WO 05/021610 of the present applicants is also partly concerned with reducing fines in the polymer, which it achieves by using a pretreatment step in the preparation of the catalyst system. Although low levels of fines are indeed achieved, we have found that the average diameter of the polymer particles produced (not disclosed in this document) was above 230 μm.
However we have found that high proportions of large polymer particles can also lead to problems, particularly with the final product (such as a pipe or film), which may contain excessive quantities of gels or white spots. In addition to being disfiguring in black and coloured compounds, the white spots may adversely influence the strength of the composition. Gel particles with a size of about 0.01-1 mm can appear as disfiguring heterogeneities in the finished film or extruded pipe. Furthermore, gels and white spots are good proxies for the degree of homogeneity in the final product: even if the gels and white spots are not themselves a problem, the associated inhomogeneity of the product may be associated with poorer physical properties. This is particularly the case in pipes for example.
Co-pending application EP 1719788A discloses bimodal polyethylene compositions having a relatively narrow particle size distribution. However the proportion of large particles in the polymer powder (as measured by D95) is higher than in the present invention, and the ratio (D90−D10)/D50 is also higher than in the present invention, where D10, D50, D90 and D95 are as defined below.